Additives are used with lubricants in order to reduce friction and increase the load carrying capacity of the lubricants. When employed in lubricants for use under extreme pressure conditions, these additives have become known as "EP" (extreme pressure additives). These lubricants are used, for example, with the type of heavy equipment used for drilling. mining, and earth moving operations and other heavy industrial applications, for example, for open and enclosed gear lubricants, house roller and rail lubricants, and for walking cam lubricants and bearing lubricants. In such cases, the base lubricants may be squeezed from between the bearing surfaces, leaving the additives to do the lubricating. These additives prevent the welding of two contacting surfaces following galling or cleavage and the production of wear fragments.
In the past, it was found that certain heavy metal soaps alone, and in combination with sulphur compounds, provided a means by which lubricating oils and greases could be made to withstand high unit loading, and to reduce wear and friction. These heavy metals primarily comprise lead, antimony, arsenic, and bismuth.
More recently, however, the use of these metal soaps has been criticized for their effect on the environment and the toxicity of these metals. Laws and regulations have been promulgated to limit the use of such metals. It has therefore become desirable, if not mandatory, to form EP additives without the use of these heavy metals, principally lead.
Recently, technology has been developed to provide the desired extreme pressure characteristics without the use of such metal soaps. For example, U.S. Pat. No. 4,259,192 discloses extreme pressure additives utilizing dithiophosphate esters of a diphosphoric acid and poly(oxalkylene)alcohol, as well as the salts of these compounds. The dithiophosphates have molecular weights ranging from 500 to 500,000, and the salts of the dithiophosphates are, for example, alkali, alkaline earth, heavy metal, and ammonium salts. However, it has been believed that under specific conditions, such as high loads and slow speeds, only lead or antimony additives can provide the desired protection.
This invention discloses new compositions to produce the protection provided by lead and antimony compounds without employing the use of such compounds. In particular, this invention provides the desired EP properties, and in addition stability in storage, while eliminating the use of environmentally dangerous and toxic heavy metals. These compositions are useful at slow speeds, such as at sliding velocities of less than 400 feet per minute, and at high loads, such as 130,000 psi, on cams, and at 50,000 to 60,000 psi at the point of contact for gears. They can also be useful at high speeds as well.
It has been thought in the past that the heavy metal compounds, such as the lead-containing materials, are effective in reducing wear and scoring by plating a minute layer of lead metal on the surfaces of the parts to be lubricated. When these parts are subjected to extreme loads greater than the base lubricants can withstand, the lead metal becomes the lubricant. Since the lead metal is soft and ductile, it forms a film between the two moving surfaces.
While it has been known in the past that copper additionally will plate out, forming the phenomenon known as "copper plating," this has only been considered to be a disadvantage. For example, U.S. Pat. No. 4,355,960 discusses the phenomenon of copper plating with regard to compressor refrigeration systems. In this case, the chlorofluorocarbon refrigerant and lubricating oils form corrosive products which dissolve copper from the refrigerating mechanism. The copper becomes redeposited onto parts of the compressor or within the fluorocarbon refrigeration systems, causing blockage of the refrigerant.
It has not been known in the past, however, to utilize this phenomenon for its lubricating abilities in extreme pressure situations.
We have found that by using proper chemical combinations, the materials subjected to the extreme pressure and temperature of heavily loaded machines will form a thin copper or molybdenum layer on the moving surfaces, which acts similarly to materials of the more toxic heavy metals, such as lead, but which does not present the same hazards. This plating presents excellent extreme pressure lubricative properties, especially under slow speeds and high loads. Under certain conditions, it is even possible to see the copper plate at the pressure point.